The improved cylinder assemblies of the present invention may be either hydraulically or pneumatically actuated. They lend themselves well, for example, to being purged of air when intended to be hydraulically actuated. For purposes of an exemplary showing, the invention will be described in its application to pneumatically actuated cylinders. It is to be understood, however, that this is not a limitation of the present invention and the teachings set forth herein apply equally well to hydraulically actuated cylinders.
The use of pneumatically actuated cylinders in industrial processes and machines is substantially unlimited. The typical cylinder assemblies on the market today are made of aluminum, stainless steel, plated steel or combinations thereof. Pneumatic cylinders of this type require air from which moisture has been removed so as to preclude internal corrosion. They also require continual lubrication which is generally accomplished by adding oil to the pressurized air.
Such conventional pneumatic cylinders are also subject to external corrosion and are generally characterized by relatively heavy weight and relatively high manufacturing, machining and material costs.
It is of utmost importance that the conventional cylinder assembly be so mounted that the longitudinal axis of the piston rod is as nearly as possible coaxial with the longitudinal axis of the cylinder. Such alignment is frequently difficult if not impossible to achieve. Misalignment, however, will cause binding, malfunction, and will destroy the front head seal, resulting in air leakage.
The present invention is based in part on the discovery that many of the above-noted deficiencies of conventional air cylinders can be avoided by making the major cylinder assembly component parts of an appropriate thermoplastic material. It has been found that ultrahigh molecular weight polyethylene will serve the purpose. Acetal resin is, in fact, preferred because of its higher strength, and its ability to withstand higher temperatures of up to about 220.degree. F. An acetal resin suitable for this purpose is manufactured by the E. I. Dupont De Nemours Company, Inc. of Wilmington, Del. under the trademark Delrin.RTM..
Thermoplastic materials of the type just described are easily machined; will hold tolerances; are resistant to oils, moisture and chemicals; can be used in temperatures of between about 180.degree. F. and about 220.degree. F.; possess high impact strength; resist wear; and are self-lubricating. These materials are also FDA approved for use in the food and drug industries. These plastic materials are about one-third the cost of the materials used in conventional cylinder assemblies and require no plating or other special finishing and polishing. Cylinder assemblies made of the above-identified thermoplastic materials, or their equivalents, demonstrate total corrosion resistance both inside and out. Cylinders made in this manner demonstrate a weight of from about one-half to about one-third the weight of conventional cylinder assemblies. Machining time to make the components is approximately one-sixth that required by conventional cylinder assemblies.
The invention is further based upon the discovery that if the cylinder assembly is provided with a floating head (i.e., a head capable of shifting perpendicular to the longitudinal axis of the cylinder in any direction throughout 360.degree.) wear of the cylinder head, the piston rod and any seals associated therewith, due to the usual misalignment problems encountered, will be greatly reduced or eliminated.
In the manufacture of most cylinder assemblies of the type with which the present application is concerned, tolerances are such that the piston and piston rod are capable of some minute rocking in any direction throughout 360.degree.. It has been found, however, that if the piston is so designed as to greatly increase its rocking capability without diminishing its continuous seal with the cylinder, wear due to misalignment is even further reduced.
It has further been determined that if the piston rod is made of an acetal resin or its equivalent, its natural resiliency will further diminish wear due to misalignment.
In use, a high percentage of conventional cylinder assemblies are destroyed and must be replaced due to misalignment wear. The practice of the present invention greatly reduces or eliminates such wear and therefore significantly lengthens the working life of the cylinder assembly.